1. Technical Field of the Invention
The present invention generally relates to the field of quadrupole mass spectrometers and, more specifically, to designs to enable precision assembly, simply and reliably, especially for small quadrupoles with filter rods less than 2" long.
2. Description of the Prior Art
Quadrupole residual gas sensors, or quadrupole mass analyzers (QMA) are well known in the art and derive from the proposal of W. Paul in 1958. Since that time, there have been many commercial implementations. In his book entitled "Quadrupole Mass Spectrometry and its Applications" (.COPYRGT.1995, American Institute of Physics, AIP Press), Peter H. Dawson summarizes much of the theory, experience and practice of quadrupole mass spectrometers and related instrumentation.
A quadrupole mass analyzer traditionally comprises an ion source, a quadrupole mass filter, and a detector, which are physically connected to each other in that order and function in that order (FIG. 1). The ion source serves to form ions of the neutral gas molecules present in the ion source and pass them through a narrow aperture to the mass filter which permits only ions of one specific mass-to-charge ratio to pass through it along the central axis through a narrow aperture to the detector. The charge that arrives at the detector is measured and is approximately proportional to the pressure of the gas species in the ion source that form that specific mass-to-charge ratio at that time. In this way, the quadrupole mass analyzer can be used to indicate the partial pressure of a particular gas species.
Typically, the mass filter can be tuned so that any mass-to-charge ratio ion within a broad range can b e measured. Thus, a QMA is typically used to monitor, sequentially, the partial pressures of a number of different gases that might be in the ion source of the analyzer. The precise behavior of the mass filter to achieve this function with good selectivity and sensitivity is somewhat complex and not the subject of this patent. However, it can be summarized as involving the interaction of combined RF and DC fields that establishes a trapping, focusing trajectory through the filter for one particular mass-to-charge ratio ion, while destabilizing and defocusing all others. In practice, the RF frequency is usually fixed, and the ratio of the RF and DC fields is also fixed to achieve a particular resolution and sensitivity for a wide range of ions. Ions of a specific mass-to-charge ratio are then selected by varying the magnitude of the RF and DC fields.
To be of practical value as a partial pressure sensor, there are several critical constraints on the mass filter. Principally, it must provide an extremely precise electric field with virtually no defects. To achieve this, the four cylindrical rods of the quadrupole mass filter must have precisely equal and uniform cross-sections and equal lengths, be positioned precisely parallel to each other with one end of each rod precisely in a common plane perpendicular to their axes and their opposite ends in another such plane. Furthermore, viewed from one end of the rods, the four rods must be positioned to occupy precisely, the four corners of a perfect square. The point at the center of the square is on the central axis of the mass filter along which the ions are filtered. The ions must be introduced to the mass filter, and extracted from it, in very small areas around this central axis. Precise alignment of all the components is, therefore, essential for good performance.
Conventional QMAs are designed with mass filter rods approximately 1/4" to 1/2" in diameter and 2 to 8 inches long. These QMAs are typically restricted to use in gas environments of less than approximately 1.E-4 torr. At higher pressures, the mean free path of the ions is too short compared to the length of the rods and the ions often cannot travel the length of the filter without colliding with other gas molecules, thereby reducing sensitivity. In recent years, there has been strong interest in extending the use of QMAs to higher pressures. This requires shorting the filter rods, significantly. This in turn requires the use of higher RF frequencies. The net effect of these and other constraints is that not only must the length be reduced, but the rod diameter and spacing also. A recent example of a small commercial instrument designed for higher pressures that is disclosed by Ferran in U.S. Pat. No. 5,613,294. This invention addresses many of the issues of small QMAs, including the loss of sensitivity that accompanies reducing the scale of the QMA, and further addresses itself to a method of manufacture that creates multiple quadrupole fields, parallel to each other, in one unit, making a quadrupole array mass analyzer (QAMA).
One serious difficulty with reducing the size of a QMA or QAMA to achieve better performance at higher pressure is achieving adequate precision of the parts, assembly of the mass filter, and assembly of the entire unit. Reducing a critical component to 1/10.sup.th of the conventional size also requires reducing the production and assembly tolerances to 1/10.sup.th of conventional levels. The practical effect of inadequate precision of an assembly such as disclosed by Ferran is addressed by Chutjian, et. al. in U.S. Pat. No. 5,719,393, which describes a manufacturing technique to improve QAMA mass filter assembly precision and corresponding performance. However, such QAMA designs employ a relatively large number of parts that must be aligned very precisely.
The present invention is directed to a design that facilitates the manufacture and assembly of a QMA, especially a small QMA, through a minimum number of parts that are substantially self-aligning. It requires virtually no assembly alignment equipment to achieve QMA units of consistently high precision and excellent performance.
The mass filter rods of conventional, large QMAs are typically supported at two or three places along their length. This is mechanically appropriate to ensure proper alignment the relatively long rods. Even the smaller QAMA disclosed by Chutjian, et. al. employs end mounting of the filter rods. Typically, multiple support mounts require special alignment fixtures and alignment procedures to achieve filter assembly to the required precision. It is a further object of this invention to eliminate this need.
In conventional QMAs, the ion source utilizes a hot filament to generate electrons that ionize the ambient gases. The filament power necessary to achieve high sensitivity in a QMA generates significant heat that must be conducted or radiated away. Furthermore, the RF losses in the dielectric supports at the ends of the rods produce more heat. The heat generated at the ion source side of the mass filter must be transferred across the mass filter region to get to the detector side of the mass filter and then to the vacuum mount and ultimately the vacuum chamber. Generally, conventional QMA designs include the mass filter rods as significant conductors of this heat. This creates a thermal gradient from one end of the mass filter to the other, often on the order of 100.degree. C. from one end of the rods to the other. Using common materials such as stainless steel for the rods and ceramic for the rod supports, this resulting thermal expansion changes the alignment of the rods from the entrance end to the exit end to a degree that is significant compared with the other tolerances of a precise QMA assembly.
Reducing this thermal distortion has been partially addressed in the work of Waki (U.S. Pat. No. 5,459,315). In that patent, Waki describes a particular way of sinking the heat generated in the dielectric supports by directly shunting it away from the dielectric. It is a further object of this invention to moderately reduce heat generation in the dielectric filter rod support and to virtually eliminate the filter rods as heat conductors, thereby eliminating thermal expansion gradients along the mass filter.
It is well known, as noted in "Quadrupole Mass Spectrometry and Its Applications" by Dawson, that the ideal cross section for a QMA mass filter electrode rod is hyperbolic. But for reasons of practical manufacture, most commercial QMAs are made using rods of circular cross section. Use of a hollow circular cylinder electrode that coaxially surrounds the mass filter reduces the sixth-order field distortion of circular rods when the inner radius of that hollow electrode is 3.54 r.sub.0, where r.sub.0 is the distance between the central axis and the nearest point on any rod and where the radius of each circular rod is 1.1468 r.sub.0. It is a further object of this invention to provide such a precise distortion-canceling field, using the minimum number of parts, without a separate electrode component or shield.
Reducing the size of a QMA and extending its application to higher pressure creates other problems also. Among these problems is a likely reduction in operating life of the ion source filament and the need for easy, more frequent replacement. It is common for QMAs to incorporate filament designs that facilitate low cost manufacture and relatively easy field replacement. However, conventional designs are typically inadequate, when reduced in scale, as required for a small high-pressure QMA. For optimum performance a reduced scale ion source can demand free spaces that are only a small fraction of a millimeter between replaceable components. It is a further object of this invention to provide an ion source with a low-cost filament assembly that allows for simple filament replacement while maintaining sub-millimeter clearances.